For the regulation of hydrostatic piston machines, adjustment devices having an adjustment piston and having a regulation valve are known, by means of which it is achieved that a torque received or output by the piston machine does not exceed a particular value or value range. This type of regulation is referred to as torque regulation, or also as power regulation, wherein the latter designation disregards the fact that the power of a machine is in fact co-determined by the rotational speed thereof. The regulation valve is referred to as torque regulator or as power regulator. Power regulation arrangements exist in the case of which the product of the pressure at the pressure port of the piston machine and the swept volume is kept constant. Here, the swept volume is the pressure medium quantity that flows through the machine per rotation of the drive shaft. In the case of other types of power regulation, a hyperbolic characteristic curve between swept volume and pressure, on which curve the torque is constant, is approximated by straight lines. In the case of such torque regulation, the torque is kept only approximately constant, wherein the adjustment device can be made simpler and more compact than an adjustment device with regulation to a constant torque.
DE 40 20 325 C2 has disclosed an adjustment device for approximate regulation to a constant torque for an adjustable hydrostatic pump, wherein the valve slide of a regulation valve, also referred to as power regulator, is displaceable in a bushing that can be driven by the adjustment piston, and said valve slide is acted on in a first displacement direction by the pump pressure and in the opposite displacement direction by a spring pack composed of two springs which are braced between the valve slide and plate springs which are adjustable but which otherwise have an inherent fixed position. In addition to the regulation valve provided for the regulation of the torque, the known adjustment device also includes a pressure regulation valve, also referred to as pressure regulator, and a delivery flow regulation valve, also referred to as delivery flow regulator. By means of each of said regulators, the inflow and outflow of control oil into the adjustment chamber and out of the adjustment chamber can be controlled at the adjustment piston. The special feature of the power regulation in this case is that the travel of the adjustment piston is fed back directly, as a travel, to the bushing of the power regulator.
U.S. Pat. No. 4,379,389 has disclosed an axial piston pump of swashplate type of construction, having a drive shaft, having a cylinder drum in which the displacement pistons are situated, having a swashplate, and having an adjustment device for power regulation. Here, the adjustment piston and the valve slide of the power regulator are arranged in alignment one behind the other. In the case of the axial piston pump known from U.S. Pat. No. 4,379,389, when the swept volume is at a maximum, the adjustment piston is deployed to the maximum extent, and the adjustment chamber at the adjustment piston has its largest volume. The valve slide is acted on by the pump pressure in a first displacement direction. In the event of an adjustment in said direction from the neutral position, control oil is discharged from the adjustment chamber and the adjustment piston retracts. Between the adjustment piston and the valve slide there are braced two feedback springs which are in the form of helical compression springs and of which a first feedback spring exerts a force on the valve slide over the entire travel of the adjustment piston, and the second feedback spring exerts a force on the valve slide only after a particular partial stroke of the adjustment piston proceeding from that position of the adjustment piston which corresponds to a maximum swept volume. In this way, a hyperbolic characteristic curve on which the torque is constant is approximated by two straight lines. In the case of an adjustment device according to U.S. Pat. No. 4,379,389, the travel of the adjustment piston is fed back, as a force, to the valve slide.
A hydrostatic axial piston machine having an adjustment device is also known from DE 100 01 826 C1. Said swashplate-type axial piston machine, designed as an axial piston pump, has a drive unit with a multiplicity of displacement pistons which are guided in cylinder bores of a cylinder drum and which, together with said cylinder bores, delimit in each case one working chamber. The displacement pistons are supported via slide shoes on a swashplate, the angle of inclination of which is variable for the purposes of varying the swept volume. Owing to the pivoting capability, an adjustable swashplate is also referred to as pivot cradle. The adjustment is performed by way of an adjustment device which has an adjustment piston which engages indirectly or directly on the pivot cradle and pivots the latter out of a basic position into which the pivot cradle is preloaded by way of an opposing piston or a spring. In the basic position, the pivot cradle may for example be set to its maximum pivot angle, in which the swept volume is at a maximum. By contrast to the axial piston pump according to U.S. Pat. No. 4,379,389, the adjustment piston is fully retracted, and the adjustment chamber has its smallest volume, when the swept volume is at a maximum. Deployment of the adjustment piston causes the pivot cradle to be pivoted back toward smaller pivot angles and smaller swept volumes.
The adjustment piston delimits an adjustment chamber which is connectable by way of a regulation valve (so-called power regulator) to a line which conducts the pump pressure or to a tank. The regulation valve has a valve slide which has the same central axis as the adjustment piston and which is preloaded by way of two feedback springs into a basic position in which the adjustment chamber is connected to the tank. To now achieve that the adjustment piston is fully retracted in the basic position, which corresponds to maximum swept volume, the feedback springs are supported on a spring rod which extends through the valve slide and is connected to the adjustment piston. The valve slide is a stepped piston with a differential surface which is acted on with the pump pressure and is arranged such that the pump pressure generates, on the valve slide, a force which is directed counter to the force of the feedback springs.
In the known solution, the two feedback springs are helical springs which are arranged coaxially with respect to one another and of which one, proceeding from a fully retracted adjustment piston and minimum adjustment chamber, acts only after a particular partial stroke and thus proceeding from a particular position of the adjustment piston on the valve slide. This yields a p-Q characteristic curve (pressure-swept volume characteristic curve) composed of two straight lines, wherein the gradient of one straight line is defined by the spring constant of the spring that is initially in engagement, and the gradient of the further straight line is defined by the sum of the spring constants of the springs which, after the partial stroke, are jointly in engagement. By means of these two straight lines that are inclined relative to one another, the hyperbolic p-Q characteristic curve, on which the torque is constant, is obtained in approximated fashion. The characteristic curve made up of two straight lines has a bend in the delivery volume, which bend corresponds to the position of the adjustment piston at which the second feedback spring begins to act.
A disadvantage of the known solution is that, owing to the spring rod which extends through the regulating piston, the adjustment device is of highly complex construction and furthermore has a considerable structural length.